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view src/share/vm/runtime/biasedLocking.hpp @ 3724:44445a59a718
Removed scripts that have been replaced by mx/commands.py.
author | Doug Simon <doug.simon@oracle.com> |
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date | Fri, 16 Dec 2011 16:47:05 +0100 |
parents | f95d63e2154a |
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/* * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved. * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. * * This code is free software; you can redistribute it and/or modify it * under the terms of the GNU General Public License version 2 only, as * published by the Free Software Foundation. * * This code is distributed in the hope that it will be useful, but WITHOUT * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License * version 2 for more details (a copy is included in the LICENSE file that * accompanied this code). * * You should have received a copy of the GNU General Public License version * 2 along with this work; if not, write to the Free Software Foundation, * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. * * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA * or visit www.oracle.com if you need additional information or have any * questions. * */ #ifndef SHARE_VM_RUNTIME_BIASEDLOCKING_HPP #define SHARE_VM_RUNTIME_BIASEDLOCKING_HPP #include "runtime/handles.hpp" #include "utilities/growableArray.hpp" // This class describes operations to implement Store-Free Biased // Locking. The high-level properties of the scheme are similar to // IBM's lock reservation, Dice-Moir-Scherer QR locks, and other biased // locking mechanisms. The principal difference is in the handling of // recursive locking which is how this technique achieves a more // efficient fast path than these other schemes. // // The basic observation is that in HotSpot's current fast locking // scheme, recursive locking (in the fast path) causes no update to // the object header. The recursion is described simply by stack // records containing a specific value (NULL). Only the last unlock by // a given thread causes an update to the object header. // // This observation, coupled with the fact that HotSpot only compiles // methods for which monitor matching is obeyed (and which therefore // can not throw IllegalMonitorStateException), implies that we can // completely eliminate modifications to the object header for // recursive locking in compiled code, and perform similar recursion // checks and throwing of IllegalMonitorStateException in the // interpreter with little or no impact on the performance of the fast // path. // // The basic algorithm is as follows (note, see below for more details // and information). A pattern in the low three bits is reserved in // the object header to indicate whether biasing of a given object's // lock is currently being done or is allowed at all. If the bias // pattern is present, the contents of the rest of the header are // either the JavaThread* of the thread to which the lock is biased, // or NULL, indicating that the lock is "anonymously biased". The // first thread which locks an anonymously biased object biases the // lock toward that thread. If another thread subsequently attempts to // lock the same object, the bias is revoked. // // Because there are no updates to the object header at all during // recursive locking while the lock is biased, the biased lock entry // code is simply a test of the object header's value. If this test // succeeds, the lock has been acquired by the thread. If this test // fails, a bit test is done to see whether the bias bit is still // set. If not, we fall back to HotSpot's original CAS-based locking // scheme. If it is set, we attempt to CAS in a bias toward this // thread. The latter operation is expected to be the rarest operation // performed on these locks. We optimistically expect the biased lock // entry to hit most of the time, and want the CAS-based fallthrough // to occur quickly in the situations where the bias has been revoked. // // Revocation of the lock's bias is fairly straightforward. We want to // restore the object's header and stack-based BasicObjectLocks and // BasicLocks to the state they would have been in had the object been // locked by HotSpot's usual fast locking scheme. To do this, we bring // the system to a safepoint and walk the stack of the thread toward // which the lock is biased. We find all of the lock records on the // stack corresponding to this object, in particular the first / // "highest" record. We fill in the highest lock record with the // object's displaced header (which is a well-known value given that // we don't maintain an identity hash nor age bits for the object // while it's in the biased state) and all other lock records with 0, // the value for recursive locks. When the safepoint is released, the // formerly-biased thread and all other threads revert back to // HotSpot's CAS-based locking. // // This scheme can not handle transfers of biases of single objects // from thread to thread efficiently, but it can handle bulk transfers // of such biases, which is a usage pattern showing up in some // applications and benchmarks. We implement "bulk rebias" and "bulk // revoke" operations using a "bias epoch" on a per-data-type basis. // If too many bias revocations are occurring for a particular data // type, the bias epoch for the data type is incremented at a // safepoint, effectively meaning that all previous biases are // invalid. The fast path locking case checks for an invalid epoch in // the object header and attempts to rebias the object with a CAS if // found, avoiding safepoints or bulk heap sweeps (the latter which // was used in a prior version of this algorithm and did not scale // well). If too many bias revocations persist, biasing is completely // disabled for the data type by resetting the prototype header to the // unbiased markOop. The fast-path locking code checks to see whether // the instance's bias pattern differs from the prototype header's and // causes the bias to be revoked without reaching a safepoint or, // again, a bulk heap sweep. // Biased locking counters class BiasedLockingCounters VALUE_OBJ_CLASS_SPEC { private: int _total_entry_count; int _biased_lock_entry_count; int _anonymously_biased_lock_entry_count; int _rebiased_lock_entry_count; int _revoked_lock_entry_count; int _fast_path_entry_count; int _slow_path_entry_count; public: BiasedLockingCounters() : _total_entry_count(0), _biased_lock_entry_count(0), _anonymously_biased_lock_entry_count(0), _rebiased_lock_entry_count(0), _revoked_lock_entry_count(0), _fast_path_entry_count(0), _slow_path_entry_count(0) {} int slow_path_entry_count(); // Compute this field if necessary int* total_entry_count_addr() { return &_total_entry_count; } int* biased_lock_entry_count_addr() { return &_biased_lock_entry_count; } int* anonymously_biased_lock_entry_count_addr() { return &_anonymously_biased_lock_entry_count; } int* rebiased_lock_entry_count_addr() { return &_rebiased_lock_entry_count; } int* revoked_lock_entry_count_addr() { return &_revoked_lock_entry_count; } int* fast_path_entry_count_addr() { return &_fast_path_entry_count; } int* slow_path_entry_count_addr() { return &_slow_path_entry_count; } bool nonzero() { return _total_entry_count > 0; } void print_on(outputStream* st); void print() { print_on(tty); } }; class BiasedLocking : AllStatic { private: static BiasedLockingCounters _counters; public: static int* total_entry_count_addr(); static int* biased_lock_entry_count_addr(); static int* anonymously_biased_lock_entry_count_addr(); static int* rebiased_lock_entry_count_addr(); static int* revoked_lock_entry_count_addr(); static int* fast_path_entry_count_addr(); static int* slow_path_entry_count_addr(); enum Condition { NOT_BIASED = 1, BIAS_REVOKED = 2, BIAS_REVOKED_AND_REBIASED = 3 }; // This initialization routine should only be called once and // schedules a PeriodicTask to turn on biased locking a few seconds // into the VM run to avoid startup time regressions static void init(); // This provides a global switch for leaving biased locking disabled // for the first part of a run and enabling it later static bool enabled(); // This should be called by JavaThreads to revoke the bias of an object static Condition revoke_and_rebias(Handle obj, bool attempt_rebias, TRAPS); // These do not allow rebiasing; they are used by deoptimization to // ensure that monitors on the stack can be migrated static void revoke(GrowableArray<Handle>* objs); static void revoke_at_safepoint(Handle obj); static void revoke_at_safepoint(GrowableArray<Handle>* objs); static void print_counters() { _counters.print(); } static BiasedLockingCounters* counters() { return &_counters; } // These routines are GC-related and should not be called by end // users. GCs which do not do preservation of mark words do not need // to call these routines. static void preserve_marks(); static void restore_marks(); }; #endif // SHARE_VM_RUNTIME_BIASEDLOCKING_HPP